A protein complex contained in the immunogenic ribonucleic acid extracts of virulent Salmonella typhimurium SR-11 stimulates protective host immunity to challenge infection as well as concomitant delayed hypersensitivity in mice almost to the exclusion of antibody formation, except that a cytophilic macroglobulin is produced which reacts with the protein complex. Comparable protein-rich extracts of the RIA vaccine strain of S. typhimurium stimulate somewhat greater resistance to lethal infection by the virulent S. typhimurium SR-11 than do the SR-11 extracts. Although both types of extracts induce delayed hypersensitivity in mice, differences in antigenic specificities of these immuno@ens are noted in in vitro assays of cell-mediated immunity. The overall objective of this research is to determine whether only the proteins (and of these, whether a specific protein) contained in these salmonella subfractions are (is) the immunogenic moieties(y) responsible for stimulating host resistance and cell-mediated immunaty. The proteins contained in the salmonella subfractions are being separated on both agarosebound enzyme columns and by ion exchange chromatography. Hopefully, quantitative and/or qualitative differences in protein content will be found which may explain the superior immunogenicity of the protein-rich extracts of the vaccine strain of S. typhimurium as well as the differences in immunologic specificity between the RIA and SR-11 extracts seen in in vitro assays of cell-mediated immunity. Moreover, the relationship between delayed hypersensitivity and protective immunity to this particular facultative intracellular parasite may be further clarified by determining the effect of anti-T-lymphocyte (anti-O) serum on the microbicidal activity of peritoneal exudate cells of immune mice on virulent S. typhimurium SR-11.